The present invention relates to an improvement in performance of a non-aqueous electrolyte lithium secondary battery comprising a negative electrode of a lithium-titanium oxide.
With the recent rapid development of technology in the field of electronics, electronic appliances have been miniaturized remarkably, and the demands for batteries of smaller size and lighter weight with a high energy density are increasing accordingly. As such batteries, lithium secondary batteries configured with a lithium negative electrode are attracting attention in this field of art, and researches and developments of these batteries are now briskly conducted in global scale. However, if metallic lithium is utilized in the negative electrode, the lithium may sometimes be pulverized and arborescent crystals called lithium dendrites may sometimes be formed on the lithium negative electrode with repetition of charge and discharge. In such cases, the shape of the lithium negative electrode may be damaged and a satisfactory charge/discharge cycle life cannot be obtained, and in an extreme case, the lithium dendrites pierce through a separator and cause an inner short-circuiting of the battery.
As a means for solving the above-mentioned inconvenience, an investigation is now conducted for a negative electrode which comprises a lithium-aluminum alloy, a lead-containing alloy capable of intercalating and deintercalating lithium, various carbon materials, an oxide of transition metal such as niobium pentoxide, an anatase-type titanium oxide, tungsten dioxide, or a ferrous or ferric oxide, each doped with lithium. For preventing against the lithium dendrite formation, carbon or an oxide of transition metal is excellent in particular, and it has been found that the charge/discharge cycle life of the battery can greatly be improved by employing these materials in the negative electrode.
Recently, lithium secondary batteries are used in various appliances as a direct-current power source for a mobile unit, a power source for memory back-up and the like. In order to fully satisfy the required performances, it is extremely important to fulfill not only sufficient energy density or charge/discharge cycle life characteristic, but also high reliability of the battery, in particular, withstanding overcharge and overdischarge characteristics of the battery. However, a combination of the above-specified negative electrode and a conventional positive electrode may not necessarily give a battery that has the withstanding overcharge and overdischarge characteristics.